The present invention relates to chemically modified liquid polymer compositions having a relatively low molecular weight of between about 1,000 and about 4,000 which, surprisingly cure to elastomers having excellent thermal stability. Certain of these chemically modified liquid polymer compositions when cured to solid elastomers also have surprisingly high tensile strength, and excellent fuel resistance. In addition, and even more surprisingly, the adhesion of such cured compositions improves after being in contact with fuel. The polymer compositions of the present invention are produced by cleaving some of the disulfide linkages occurring in certain mercaptan terminated liquid polymers with certain organic dimercaptan compounds in the presence of an organic amine catalyst.
Polymers having disulfide linkages are often times referred to as polysulfides which are commercially available under the name Thiokol polysulfides. Such polysulfide polymers are disclosed in U.S. Pat. No. 2,466,963 which issued in 1949. Certain polymers prepared according to this patent have molecular weights on the order of about 1,000 to about 8,000 which are viscous liquids having viscosities within the range of about 1,500 to 150,000 centipoise and have repeating units of diethyl formal disulfide with terminal mercaptan groups, such polymers being hereinafter referred to as diethyl formal mercaptan terminated polysulfides. When such polymers are cured or hardened, they form rubberlike solids having a number of excellent properties including resistance to oil, organic liquids, water, etc. Moreover, such polymers are resistant to ozone or sunlight and are relatively impermeable to gases and moisture. As a result, diethyl formal mercaptan terminated polysulfides have been used widely in sealant, adhesive and coating compositions.
Although the diethyl formal mercaptan terminated polysulfides can be cured to provide useful products, there is a distinct need in the art to improve upon various properties of the liquid diethyl formal mercaptan terminated polysulfides and to provide new compositions which have valuable properties and characteristics not exhibited by the available diethyl formal mercaptan terminated polysulfides.
For example, the low molecular weight, low viscosity diethyl formal mercaptan terminated polysulfides cure only to elastomers having poor thermal stability, low tensile strength and poor adhesion. The higher molecular weight, higher viscosity diethyl formal mercaptan terminated polysulfides, although not having good thermal stability, do exhibit other good properties, such as good adhesion and good tensile strength when cured. However, these properties are only achieved with diethyl formal mercaptan polysulfides having such high viscosities that it is necessary to use solvents to lower the viscosity to facilitate their use in adhesives, sealants and coatings. In addition, such high viscosities limit the amount of additives, such as fillers, pigments and plasticizers which diethyl formal mercaptan terminated polysulfides are compounded with.
The chemically modified disulfide liquid polymer compositions of the present invention have, when cured, greatly improved thermal stability when compared to diethyl formal mercaptan terminated polysulfides having similar molecular weights and viscosities. Certain of the chemically modified disulfide liquid polymer compositions of the present invention when cured and when compared to diethyl formal mercaptan terminated polysulfides have:
1. Superior resistance to ultraviolet light;
2. Higher tensile strength;
3. Higher tear strength;
4. Superior resistance to higher temperatures;
5. Superior resistance to fuel;
6. Greater compatibility with traditional plasticizers used in compounding diethyl formal mercaptan terminated polysulfides; and, most surprisingly,
7. Greater adhesion after exposure to organic liquid such as aromatic hydrocarbon fuel.
The chemically modified disulfide polymer compositions of the present invention which have the seven properties enumerated above also do not require solvents to lower their viscosities and therefore are extremely useful in sealant, coating or adhesive formulations. The elimination of solvents is extremely advantageous because solvents are very toxic, and, moreover, pose a flammability hazard in such formulations. Such nonsolvent based elastomers are particularly beneficial in providing insulating glass sealant formulations since the stress caused by shrinkage during solvent evaporation is eliminated. Furthermore, such low viscosity solvent free polymer compositions permit inclusion of greater amounts of reinforcing pigments and extenders without loss of other desirable properties of critical specification viscosity requirements in insulating glass, construction, and aircraft sealants. In addition, the disulfide polymer compositions of the present invention are less expensive since the added ingredients, such as plasticizers and pigments, normally cost less per gallon than diethyl formal mercaptan terminated polysulfides.